International patent application WO 88/09369 describes water-based liquid detergents for dishwashing machines which contain 0.5% to 20% by weight of a mixture of alkyl polyglycosides corresponding to the formula R--O--(Z).sub.x H, where R=C.sub.8-16 alkyl, Z=a glucose unit and x=1-3, preferably 1-2 and more particularly 1-1.4, and low-foaming nonionic surfactants corresponding to the formula R.sub.1 --O--(EO).sub.n (PO).sub.m H, where R.sub.1 =C.sub.6-22 alkyl, n=1-40 and m=1-15, and 10% to 60% by weight of alkali metal citrates. Detergents such as these are unsuitable as rinse aids.
It is known that, where modern phosphate-free low-alkali detergents are used in dishwashing machines, lime and silicate coatings can be formed both on the washed articles and in the interior of the machine because the calcium binding capacity of such detergents is lower than that of conventional phosphate-containing products. Troublesome lime and silicate coatings are formed in particular when the rinsing water of the dishwashing machine is unsoftened or insufficiently softened and a water hardness of 4.degree.dH is exceeded. In cases such as these, lime and silicate coatings can be effectively avoided by the introduction of citric acid into the final rinse cycle via the rinse aid. However, since the quantities of rinse aid normally added during the final rinse cycle are very small (3 ml to 6 ml), the citric acid content in rinse aid formulations intended to guarantee the effective inhibition of bloom has to be relatively high to achieve an adequate acid or complexing capacity. High citric acid contents support the effect of phosphate substitutes and guarantee spotless dishes.
European patent application 432 836 (Unilever) describes rinse aid formulations for dishwashing machines which contain only one surfactant selected from alkyl polyglycosides and, as further ingredients, foam inhibitors and thickeners. They do not contain any of the acid normally present in rinse aids.
Foam inhibitors are an essential ingredient of these formulations because alkyl polyglycosides are generally high-foaming and would cause unacceptable foaming of the described rinse aid formulations when used in a dishwashing machine. However, foam inhibitors are only effective if they are insoluble in the medium to be foam-inhibited. Accordingly, the alkyl polyglycoside solutions and the foam inhibitors are also immiscible with one another in the disclosed examples of rinse aids. Accordingly, two phases would exist unless a thickener--which provides for a certain homogenization--had been used to disperse them.
However, the latent inhomogeneity caused by the foam inhibitor remains a disadvantage of such a formulation. The foam inhibitor and surfactant solution of the rinse aid separate after only a short storage time, despite the content of thickener. A product such as this is of course unsuitable both for consumers of branded goods and for bulk consumers because the products required in both these cases are required to remain stable in storage over prolonged periods, but at least for one year. Accordingly, the only suitable formulations are single-phase formulations in which all the ingredients are homogeneously dissolved and which not only remain stable in storage, but also do not separate during the heating and cooling phase of the rinse cycle, i.e. show phase stability at temperatures in the dishwashing machine of 0.degree. to 65.degree. C. The low temperature is necessary because warehouses are generally not heated in winter. Accordingly, temperatures around freezing point are entirely realistic. On the other hand, the known products are so viscous--as long as they are homogeneous--that they cannot be poured in through the rinse aid dispenser of a domestic dishwashing machine.
In addition, rinse aid formulations containing only alkyl polyglycosides as their surfactant component do not wet plastic articles sufficiently, if at all, so that the clear rinse or clear drying effect is unsatisfactory in their case.
Today, only those formulation ingredients which are totally biodegradable under detergent legislation may be used in detergents, including rinse aid formulations. Accordingly, the problem addressed by the present invention was to find an ecologically and toxicologically satisfactory formulation which would be equivalent in its performance properties to commercial rinse aids and which would not have any of the disadvantages mentioned above.